Electric resistance welded steel pipe is utilized in a wide range of fields, including gasoline and natural gas line pipes, oil well pipes, and nuclear, geothermal, chemical plant, mechanical structural, and general pipes. When manufacturing electric resistance welded steel pipe, a strip shaped steel plate such as a hot-rolled steel strip is formed into a pipe shape, and heated and melted by a high frequency current while converging abutting edge faces in a V-shape, to form a weld seam. Weld defects occur in electric resistance welding when heat input power, welding speed and the like are not controlled within appropriate ranges. For example, non-welded portions sometimes occur in cases in which the heat input is insufficient, or the welding speed is fast. Moreover, a large quantity of oxide sometimes remains on the welded portion in cases in which the heat input is excessive, or the welding speed is slow.
Welding conditions during electric resistance welding are generally broadly divided into three types of welding condition: a type 1 welding condition with small positional fluctuation of a welding point where the end faces of a metal plate first contact; a type 2 welding condition with intermediate positional fluctuation amplitude and positional fluctuation cycle of the welding point; and a type 3 welding condition with large positional fluctuation amplitude and positional fluctuation cycle of the welding point. In fast welding speed cases, if the heat input is increased, a type 2 welding condition accompanied by two-phased reduction of the Vee angle exists, that differs from the type 1 welding condition, the type 2 welding condition, and the type 3 welding condition.
The two-phased reduction of the Vee angle type 2 welding condition is a welding condition with an intermediate positional fluctuation amplitude and positional fluctuation cycle of the welding point, similarly to the type 2 welding condition, and the weld portion forms a two-stage V shape.
FIG. 20 schematically illustrates relationships between welding speed and heat input for each type of welding condition. In FIG. 20, region 2001 corresponds to the type 1 welding condition, region 2002 corresponds to the type 2 welding condition, region 2003 corresponds to the type 3 welding condition, and region 2004 corresponds to the two-phased reduction of the Vee angle type 2 welding condition. Vm is the critical welding speed at which the two-phased reduction of the Vee angle type 2 welding condition appears, and Tm is the melting point of the steel plate.
When the welding speed is below the critical welding speed Vm and the heat input is low, the welding condition becomes the type 1 welding condition. When the heat input is increased, even when the welding speed is below the critical welding speed Vm, the welding condition becomes the type 2 welding condition, and transitions to the type 3 welding condition when the heat input is increased further. However, when the welding speed is the critical welding speed Vm or greater, the welding condition transitions from the type 1 welding condition to the type 2 welding condition accompanying an increase in the heat input, and when the heat input is increased further, becomes the two-phased reduction of the Vee angle type 2 welding condition.
In the type 1 welding condition, there is a possibility of being unable to melt the steel plate over the entire thickness direction of the circumferential direction edge portions of the steel plate that are abutted against each other. In the type 3 welding condition, there is a possibility of a large quantity of oxide remaining on the weld portion due to overheating of the circumferential direction edge portions of the steel plate that are abutted against each other. Moreover, although the type 2 welding condition enables the steel plate to be melted over the entire thickness direction of the circumferential direction edge portions of the steel plate that are abutted against each other, there is concern that a region might develop to a state at the boundary with the two-phased reduction of the Vee angle type 2 welding condition, in which oxide remains on the weld portion. There is also a possibility that the range of the type 2 welding condition narrows due to the effects of, for example, variation in forming. Such cases make it difficult to control heat input to stay within the range of the type 2 welding condition, it is desirable to perform electric resistance welding in a two-phased reduction of the Vee angle type 2 welding condition with wider range.
Technology for performing electric resistance welding in the two-phased reduction of the Vee angle type 2 welding condition is described in Japanese Patent Application Laid-Open (JP-A) No. H04-231181 and International Publication (WO) No. 2011/118560.
JP-A No. H04-231181 describes employing a vibration amplitude Δf of a signal obtained by F/V conversion of fine fluctuations in the output frequency of a power source supplied to perform electric resistance welding, together with a fluctuation count SPL per time unit of the signal, to create conditions to give a welding condition of the two-phased reduction of the Vee angle type 2 welding condition, and outputting a heat input power that satisfies the conditions, as a heat input control reference signal.